fix(docs): tutorial source-plugin -> fix code block language, add title

[muescha]

Description

changes:

• fix language: javascript -> jsx
• add title for consistency in this tutorial: all shell in subdir have a title

Related Issues

• docs: new source plugin tutorial #23363 docs: new source plugin tutorial

[Ekwuno]

Thanks for sending this in. I left a few suggestions.

[Ekwuno]

I wonder if this can be done for every shell command on the Gatsby website? I think not. I would rather leave a shell tag without a title because the text before it implies running it in the example-site directory.

Suggested change

	```shell:title=example-site
	```shell

[muescha]

gatsby/docs/tutorial/source-plugin-tutorial.md

Line 439 in 2630758

```shell:title=source-plugin

gatsby/docs/tutorial/source-plugin-tutorial.md

Line 534 in 2630758

```shell:title=example-site

if you look into the full tutorial text: the shell commands in this tutorial use this title to mark the directory if it is in a subdirectory - only this 2 places was not.

to be consistent in one doc there are 2 solutions:

1. remove it from the other shell, or
2. add it here.

i would add it here because that helps a little the reader - i liked here that also the js code examples had the subdirs are in the title :)

[Ekwuno]

I think kyle was trying to be more specific because it wasn't implied in the two places the :title=example-site tag was used with shell. The other 4 places were implied.

[muescha]

gatsby/docs/tutorial/source-plugin-tutorial.md

Lines 437 to 439 in 2630758

	Start by installing `gatsby-source-filesystem` in the `source-plugin` project:
	```shell:title=source-plugin

gatsby/docs/tutorial/source-plugin-tutorial.md

Lines 532 to 534 in 2630758

	Install `gatsby-plugin-sharp` and `gatsby-transformer-sharp` in the `example-site` (_not_ the plugin):
	```shell:title=example-site

in the other 2 cases it is noted also the line before.

but i think, more easy for the reader is if he reads a second time in the title in which directory he should execute the commands...

[Ekwuno]

Suggested change

	```shell:title=source-plugin
	```shell

[Ekwuno]

This looks good to me.

[marcysutton]

Looks good to me. Thanks for the PR!