a bunch of grammar fixes

src/lib/utils/source.c

@@ -99,7 +99,7 @@ Source *Source_FromFile(const char *file, Error *error)
 		return NULL;
 	}
 
-	// Open the file and get it's size.
+	// Open the file and get its size.
 	// at the end of the block, the file
 	// cursor will point at the start of
 	// the file.

src/lib/utils/utf8.c

@@ -243,7 +243,7 @@ int utf8_sequence_to_utf32_codepoint(const char *utf8_data, int nbytes, uint32_t
 **       value is equal to [nbytes].
 **
 ** NOTE: You can check the validity of a UTF-8 string
-**       by calling this function and checking that it's
+**       by calling this function and checking that its
 **       return value is not negative.
 */
 int utf8_strlen(const char *utf8_data, int nbytes)