docs: fix typo in the "Shared cache" section (#15346)

### What does this PR try to resolve?
This PR addresses a typo in the "Shared cache" section. The word
"environmental" has been corrected to "environment" for proper grammar
in the phrase:
- "set `RUSTC_WRAPPER` environmental variable to `sccache`" → "set
`RUSTC_WRAPPER` environment variable to `sccache`".

### How should we test and review this PR?
Review the change in the relevant documentation section and verify that
the typo has been corrected. No functional changes are involved.

### Additional information
This is a simple grammar fix to ensure clarity in the documentation.

Author: epage

src/doc/src/reference/build-cache.md

@@ -83,7 +83,7 @@ A third party tool, [sccache], can be used to share built dependencies across
 different workspaces.
 
 To setup `sccache`, install it with `cargo install sccache` and set
-`RUSTC_WRAPPER` environmental variable to `sccache` before invoking Cargo. If
+`RUSTC_WRAPPER` environment variable to `sccache` before invoking Cargo. If
 you use bash, it makes sense to add `export RUSTC_WRAPPER=sccache` to
 `.bashrc`. Alternatively, you can set [`build.rustc-wrapper`] in the [Cargo
 configuration][config]. Refer to sccache documentation for more details.