Fix invalid definition of strlen in error codes documentation

Author: Urgau

compiler/rustc_error_codes/src/error_codes/E0755.md

@@ -20,7 +20,7 @@ side effects or infinite loops:
 
 extern "C" {
     #[unsafe(ffi_pure)] // ok!
-    pub fn strlen(s: *const i8) -> isize;
+    pub fn strlen(s: *const std::ffi::c_char) -> usize;
 }
 # fn main() {}
 ```

compiler/rustc_error_codes/src/error_codes/E0756.md

@@ -21,7 +21,7 @@ which have no side effects except for their return value:
 
 extern "C" {
     #[unsafe(ffi_const)] // ok!
-    pub fn strlen(s: *const i8) -> i32;
+    pub fn strlen(s: *const std::ffi::c_char) -> usize;
 }
 # fn main() {}
 ```