build: fix LINT_MD_NEWER assignment

Indentation with a tab breaks the functionality, resulting in linting
all .md files when any one is changed. For consistency with the rest of
the Makefile and to restore functionality, remove indentation.

Refs: #32614 (comment)

PR-URL: #32712
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

Makefile

@@ -1229,9 +1229,9 @@ lint-md-build:
 	$(warning "Deprecated no-op target 'lint-md-build'")
 
 ifeq ("$(wildcard tools/.mdlintstamp)","")
-	LINT_MD_NEWER =
+LINT_MD_NEWER =
 else
-	LINT_MD_NEWER = -newer tools/.mdlintstamp
+LINT_MD_NEWER = -newer tools/.mdlintstamp
 endif
 
 LINT_MD_TARGETS = doc src lib benchmark test tools/doc tools/icu $(wildcard *.md)