Update assignment section "=" (#1626)

I think these are more informative and precise examples for assignment expression.

makefile.md

@@ -8,10 +8,13 @@ category: CLI
 ## Var assignment
 
 ```makefile
-uglify = $(uglify)        # lazy assignment
-compressor := $(uglify)   # immediate assignment
-prefix ?= /usr/local      # safe assignment
-hello += world            # append
+foo  = "bar"
+bar  = $(foo) foo  # dynamic (renewing) assignment
+foo := "boo"       # one time assignment, $(bar) now is "boo foo"
+foo ?= /usr/local  # safe assignment, $(foo) and $(bar) still the same
+bar += world       # append, "boo foo world"
+foo != echo fooo   # exec shell command and assign to foo
+# $(bar) now is "fooo foo world"
 ```
 
 `=` expressions are only evaluated when they're being used.
@@ -98,7 +101,6 @@ make
   -e, --environment-overrides
   -B, --always-make
   -s, --silent
-
   -j, --jobs=N   # parallel processing
 ```
 
@@ -107,9 +109,9 @@ make
 ```makefile
 foo: $(objects)
 ifeq ($(CC),gcc)
-        $(CC) -o foo $(objects) $(libs_for_gcc)
+  $(CC) -o foo $(objects) $(libs_for_gcc)
 else
-        $(CC) -o foo $(objects) $(normal_libs)
+  $(CC) -o foo $(objects) $(normal_libs)
 endif
 ```