docs: Use explicitely the "cython" module in examples (GH-4284)

docs/examples/tutorial/external/keyword_args_call.py

-import cython
 from cython.cimports.strstr import strstr
 
 def main():
-    data: p_char = "hfvcakdfagbcffvschvxcdfgccbcfhvgcsnfxjh"
+    data: cython.p_char = "hfvcakdfagbcffvschvxcdfgccbcfhvgcsnfxjh"
 
-    pos: p_char = strstr(needle='akd', haystack=data)
+    pos = strstr(needle='akd', haystack=data)
     print(pos is not cython.NULL)

docs/examples/tutorial/external/keyword_args_call.pyx

 cdef extern from "string.h":
     char* strstr(const char *haystack, const char *needle)
 
-
 cdef char* data = "hfvcakdfagbcffvschvxcdfgccbcfhvgcsnfxjh"
 
 cdef char* pos = strstr(needle='akd', haystack=data)