g-code.txt: Minor typos spotted when reading docs

Author: andypugh

docs/src/gcode/g-code.txt

@@ -1591,8 +1591,8 @@ threading cut will be 'K' beyond the 'thread peak' position.
 Optional settings
 
 * '$-' - The spindle number to which the motion will be synchronised
-(default 0). For example is $1 is programmed then the motion will begin
-on the reset od spindle.1.index-enable and proceed in synchrony with the
+(default 0). For example if $1 is programmed then the motion will begin
+on the reset of spindle.1.index-enable and proceed in synchrony with the
 value of spindle.1.revs
 
 * 'R-' - The 'depth degression'. 'R1.0' selects constant depth on successive