user guide: small grammar fixes (#10)

2 tiny fixes:
1. `its` -> `it's`, meaning "it is"
1. `taught` (past tense of _teach_) -> `taut` (tightly drawn, tense) [link](https://www.dictionary.com/browse/taut)

Author: mbifulco

user_guide/docs/build.md

@@ -90,7 +90,7 @@ The use the rubber band to hold the panel onto the jacks and board. Place it bet
 
 ![Rubber band placed](images/kit/3a-jack-alignment.jpg)
 
-Double (or triple) it over to make sure its taught.
+Double (or triple) it over to make sure it's taut.
 
 ![Rubber band pulled taught](images/kit/3b-jack-alignment.jpg)