Avoid deprecated function in example in README.

Fraser999 · Fraser999 · commit c450b1ee846a · 2016-08-19T11:20:21.000+01:00
diff --git a/README.md b/README.md
@@ -12,7 +12,7 @@ library is located.
 extern crate pkg_config;
 
 fn main() {
-    pkg_config::find_library("foo").unwrap();
+    pkg_config::probe_library("foo").unwrap();
 }
 ```
 

Original file line number	Diff line number	Diff line change
`@@ -12,7 +12,7 @@ library is located.`
`12`	`12`	`extern crate pkg_config;`
`13`	`13`
`14`	`14`	`fn main() {`
`15`		`- pkg_config::find_library("foo").unwrap();`
	`15`	`+ pkg_config::probe_library("foo").unwrap();`
`16`	`16`	`}`
`17`	`17`	```
`18`	`18`