Fix casing: "package URL" -> "Package URL" at the beginning of sentence

In an ExternalIdentifierType entry.

Signed-off-by: Arthit Suriyawongkul <arthit@gmail.com>

Signed-off-by: Arthit Suriyawongkul <arthit@gmail.com>

model/Core/Vocabularies/ExternalIdentifierType.md

@@ -22,7 +22,7 @@ ExteralIdentifierType specifies the type of an external identifier.
 - email: Email address, as defined in [RFC 3696](https://www.rfc-editor.org/info/rfc3986) Section 3.
 - gitoid: [Gitoid](https://www.iana.org/assignments/uri-schemes/prov/gitoid), stands for [Git Object ID](https://git-scm.com/book/en/v2/Git-Internals-Git-Objects). A gitoid of type blob is a unique hash of a binary artifact. A gitoid may represent either an [Artifact Identifier](https://github.com/omnibor/spec/blob/eb1ee5c961c16215eb8709b2975d193a2007a35d/spec/SPEC.md#artifact-identifier-types) for the software artifact or an [Input Manifest Identifier](https://github.com/omnibor/spec/blob/eb1ee5c961c16215eb8709b2975d193a2007a35d/spec/SPEC.md#input-manifest-identifier) for the software artifact's associated [Artifact Input Manifest](https://github.com/omnibor/spec/blob/eb1ee5c961c16215eb8709b2975d193a2007a35d/spec/SPEC.md#artifact-input-manifest); this ambiguity exists because the Artifact Input Manifest is itself an artifact, and the gitoid of that artifact is its valid identifier. Gitoids calculated on software artifacts (Snippet, File, or Package Elements) should be recorded in the SPDX 3.0 SoftwareArtifact's contentIdentifier property. Gitoids calculated on the Artifact Input Manifest (Input Manifest Identifier) should be recorded in the SPDX 3.0 Element's externalIdentifier property. See [OmniBOR Specification](https://github.com/omnibor/spec/), a minimalistic specification for describing software [Artifact Dependency Graphs](https://github.com/omnibor/spec/blob/eb1ee5c961c16215eb8709b2975d193a2007a35d/spec/SPEC.md#artifact-dependency-graph-adg).
 - other: Used when the type does not match any of the other options.
-- packageUrl: [package URL](../../../annexes/pkg-url-specification.md)
+- packageUrl: [Package URL](../../../annexes/pkg-url-specification.md)
 - securityOther: Used when there is a security related identifier of unspecified type.
 - swhid: SoftWare Hash IDentifier, persistent intrinsic identifiers for digital artifacts, such as files, trees (also known as directories or folders), commits, and other objects typically found in version control systems. The syntax of the identifiers is defined in the [SWHID specification](https://www.swhid.org/specification/v1.1/4.Syntax) and they typically look like `swh:1:cnt:94a9ed024d3859793618152ea559a168bbcbb5e2`.
 - swid: Concise Software Identification (CoSWID) tag, as defined in [RFC 9393](https://www.rfc-editor.org/info/rfc9393) Section 2.3.