languages: Fix detents case line after typing : in Python (#34017)

Closes #34002

`decrease_indent_patterns` should only contain mapping which are at same
indent level with each other, which is not true for `match` and `case`
mapping.

Caused in https://github.com/zed-industries/zed/pull/33370

Release Notes:

- N/A

crates/languages/src/python/indents.scm

@@ -14,4 +14,4 @@
 (else_clause) @start.else
 (except_clause) @start.except
 (finally_clause) @start.finally
-(case_pattern) @start.case
+(case_clause) @start.case