os: fix typo in comment (Readir -> Readdir)
os: fix typo in comment (Readir -> Readdir)

Change-Id: I8434925661dc11396380af65c192c9f0dc191287
Reviewed-on: https://go-review.googlesource.com/27973
Reviewed-by: Brad Fitzpatrick <ae9783c0b0efc69cd85ab025ddd17aa44cdc4aa5@golang.org>

nawawi/go,ImJasonH/go,vsekhar/elastic-go,momchil-velikov/go,akutz/go,derekmarcotte/go,akutz/go,suncycheng/go,polyverse-security/go,derekmarcotte/go,linux-on-ibm-z/go,polyverse-security/go,CAFxX/go,ImJasonH/go,josharian/go,CAFxX/go,ImJasonH/go,akutz/go,ImJasonH/go,nawawi/go,polyverse-security/go,Cofyc/go,Cofyc/go,momchil-velikov/go,goodlang/good,akutz/go,linux-on-ibm-z/go,codestation/go,suncycheng/go,vsekhar/elastic-go,momchil-velikov/go,goodlang/good,golang/go,cloudflare/go,golang/go,suncycheng/go,goodlang/good,polyverse-security/go,vsekhar/elastic-go,golang/go,suncycheng/go,codestation/go,Cofyc/go,polyverse-security/go,golang/go,josharian/go,linux-on-ibm-z/go,CAFxX/go,akutz/go,vsekhar/elastic-go,ImJasonH/go,goodlang/good,CAFxX/go,codestation/go,goodlang/good,goodlang/good,josharian/go,goodlang/good,CAFxX/go,suncycheng/go,nawawi/go,codestation/go,CAFxX/go,ImJasonH/go,ImJasonH/go,vsekhar/elastic-go,linux-on-ibm-z/go,akutz/go,golang/go,CAFxX/go,cloudflare/go,christopher-henderson/Go,vsekhar/elastic-go,nawawi/go,Cofyc/go,Cofyc/go,derekmarcotte/go,linux-on-ibm-z/go,codestation/go,vsekhar/elastic-go,golang/go,akutz/go,golang/go,vsekhar/elastic-go,derekmarcotte/go,linux-on-ibm-z/go,ImJasonH/go,polyverse-security/go,cloudflare/go,momchil-velikov/go,josharian/go,nawawi/go,codestation/go,Cofyc/go,momchil-velikov/go,cloudflare/go,momchil-velikov/go,momchil-velikov/go,cloudflare/go,CAFxX/go,goodlang/good,linux-on-ibm-z/go,suncycheng/go,nawawi/go,cloudflare/go,Cofyc/go,codestation/go,codestation/go,josharian/go,derekmarcotte/go,derekmarcotte/go,derekmarcotte/go,polyverse-security/go,Cofyc/go,nawawi/go,cloudflare/go,suncycheng/go,derekmarcotte/go,josharian/go,golang/go,polyverse-security/go,suncycheng/go,cloudflare/go,linux-on-ibm-z/go,nawawi/go,josharian/go,momchil-velikov/go,josharian/go,akutz/go