fix README.md incorrect cmake function name

Author: akhilharihar

README.md

@@ -399,7 +399,7 @@ You can use
 ```cmake
 pico_set_program_name(foo "not foo") # as "foo" would be the default
 pico_set_program_description(foo "this is a foo")
-pico_set_program_version_string(foo "0.00001a")
+pico_set_program_version(foo "0.00001a")
 pico_set_program_url(foo "www.plinth.com/foo")
 ```