[12.x] Fix anchor format for toPrettyJson

[AhmedAlaa4611]

Description

This PR updates the recently added toPrettyJson link to use the slug format.

In the past, I attempted a full unification, but that PR was closed due to the broad impact on existing links across the web. This PR limits the change to only this new method, which was just merged a few hours ago.

Currently, the documentation contains a mix of anchor formats. I think we can stick with the slug format for new methods to ensure future consistency.

[shaedrich]

I don't really see why one link on that particular page should look different from all the others.

[AhmedAlaa4611]

I don't really see why one link on that particular page should look different from all the others.

As I said:

Currently, the documentation contains a mix of anchor formats ...

See the following links:

docs/collections.md

Line 143 in a2152c2

[firstOrFail](#method-first-or-fail)

docs/collections.md

Line 144 in a2152c2

[firstWhere](#method-first-where)

docs/collections.md

Line 199 in a2152c2

[reduceSpread](#method-reduce-spread)

docs/collections.md

Line 123 in a2152c2

[countBy](#method-countBy)

docs/collections.md

Line 121 in a2152c2

[containsStrict](#method-containsstrict)

I just try to unify the newly introduced links.