Fix minor typo

From `Ths function is called...` to `This function is called`.

exercises/practice/hello-world/hello_world.h

@@ -7,7 +7,7 @@
 // Declare the 'hello()' function, which takes no arguments and returns a
 // 'const char *', i.e. a pointer to a character (in this case the first
 // character in a string). The function itself is defined in the hello_world.c
-// source file. Ths function is called by the test case(s) in the test source
+// source file. This function is called by the test case(s) in the test source
 // file test/test_hello_world.c.
 const char *hello(void);