libexpr/parser: Fix small error position mistake

In one of the cases it pointed to the first instead of the second
occurence, which is a bit confusing

Change-Id: Ie508d1f84feb434708804d45aa8a9c8b1e3c5f69

tests/functional2/lang/parser-merge-attrs/parse-fail-err1.err.exp

@@ -1,6 +1,6 @@
-error: attribute 'z' already defined at /pwd/in.nix:3:16
-       at /pwd/in.nix:2:3:
-            1| {
+error: attribute 'z' already defined at /pwd/in.nix:2:3
+       at /pwd/in.nix:3:16:
             2|   x.z = 3;
-             |   ^
             3|   x = { y = 3; z = 3; };
+             |                ^
+            4| }

tests/functional2/lang/parser-merge-attrs/parse-fail-err2.err.exp

@@ -1,6 +1,6 @@
-error: attribute 'y' already defined at /pwd/in.nix:3:9
-       at /pwd/in.nix:2:3:
-            1| {
+error: attribute 'y' already defined at /pwd/in.nix:2:3
+       at /pwd/in.nix:3:9:
             2|   x.y.y = 3;
-             |   ^
             3|   x = { y.y= 3; z = 3; };
+             |         ^
+            4| }