Correct error in 07 variable assignments.md

[skywalkerylh]

good resources for learning yocto!

But In the file 07 variable assignments.md, there is a small error in the example for variable assignments.

Current Content

# Append
A ?= "val"
A += "var"

# The final value is A="var"

Suggestioin

# Append
A ?= "val"
A += "var"

# The final value is A="val var"

[skywalkerylh]

another correction

A = "foo bar"
A:remove = "var"
A += "var"
The final value is A=" foo bar "