[Doc] Update PROTOCOL-HTTP2.md (#34907)

more strict  --> stricter

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Closes #34907

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/34907 from lixin963:patch-1 9081eb445e
PiperOrigin-RevId: 591176601

doc/PROTOCOL-HTTP2.md

@@ -75,7 +75,7 @@ Base64-encoded values.
 
 **ASCII-Value** should not have leading or trailing whitespace. If it contains
 leading or trailing whitespace, it may be stripped. The **ASCII-Value**
-character range defined is more strict than HTTP. Implementations must not error
+character range defined is stricter than HTTP. Implementations must not error
 due to receiving an invalid **ASCII-Value** that's a valid **field-value** in
 HTTP, but the precise behavior is not strictly defined: they may throw the value
 away or accept the value. If accepted, care must be taken to make sure that the