docs: fix typo (#3)

Author: FlyingCaiChong

README.md

@@ -45,7 +45,7 @@ s.original // 'var issues = 99' (applied with previous changes)
 s.replace('issues', 'problems')
 s.toString() // 'var problems = 99'
 
-s.generateMap() // generate sourcemap, if multiple commits happend, it will generate a combined sourcemap
+s.generateMap() // generate sourcemap, if multiple commits happened, it will generate a combined sourcemap
 ```
 
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