fix: convDate should be the right side of an assignment (#22)

**Description**

This PR modifies the grammar to have convDate as the RHS of an
assignment

Author: LeanSerra

examples/convDate.lm

@@ -3,5 +3,5 @@ main(){
         a, b, c : int
     }
 
-    convDate(11-11-1900)
+    a := convDate(11-11-1900)
 }

inputs/test.txt

@@ -56,4 +56,4 @@ read(f6f)
 #+ x := 1001247380987123904721309473201974012938740921374091237409127340983217094874.1234 +#
 
 
-convDate(01-02-1900)
+a1 := convDate(01-02-1900)