docs: match npm-exec.md -p usage with lib/exec.js

updated usage from: https://github.com/npm/cli/blob/v7.0.1/lib/exec.js#L8-L20,
and removed extra `-p` occurrences to avoid confusion.

Credit: @dr-js
Close: #1970
Reviewed-by: @ruyadorno

docs/content/commands/npm-exec.md

@@ -30,9 +30,9 @@ This command allows you to run an arbitrary command from an npm package
 (either one installed locally, or fetched remotely), in a similar context
 as running it via `npm run`.
 
-Whatever packages are specified by the `--package` or `-p` option will be
+Whatever packages are specified by the `--package` option will be
 provided in the `PATH` of the executed command, along with any locally
-installed package executables. The `--package` or `-p` option may be
+installed package executables. The `--package` option may be
 specified multiple times, to execute the supplied command in an environment
 where all specified packages are available.
 
@@ -48,7 +48,7 @@ only be considered a match if they have the exact same name and version as
 the local dependency.
 
 If no `-c` or `--call` option is provided, then the positional arguments
-are used to generate the command string. If no `-p` or `--package` options
+are used to generate the command string. If no `--package` options
 are provided, then npm will attempt to determine the executable name from
 the package specifier provided as the first positional argument according
 to the following heuristic: