Correction in custom validation example of Guide.md
Correction in custom validation example of Guide.md

If only Full name is allowed then error should be thrown when the value is not equal to 'Full name'.
abernix/meteor-useraccounts-core,dandv/core,meteor-useraccounts/core,jonxduo/core,logankoester/core,gevou/core,PhilippSpo/core,gevou/core,rawGem/ua-core,gwendall/core,jjeffwarner/core,phocks/core,logankoester/core,evgenTraytyak/core,niole/core,rawGem/aus-core,lucaslizama/core,jjeffwarner/core,challett/core,jxm262/core,meteor-useraccounts/core,evgenTraytyak/core,lucaslizama/core,phocks/core,niole/core,ShawnOceanHu/core