Typo fix: firestore_.md

[abhigyadufare]

Hey there! So you want to contribute to a Firebase SDK?
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[egilmorez]

Good catch!

[hsubox76]

These files are autogenerated and the text has to be changed in the source code. Same with #7336. It can be tricky to find the right source file so if you can't find it, you can file an issue for both of these and someone on the team can try to pinpoint it.