[gthumb] readme: updated the build instructions

[Paolo Bacchilega <paobac src gnome org>]

commit 98aba8de0ad01322b4451cdbe150874d1e7d80af
Author: Paolo Bacchilega <paobac src gnome org>
Date:   Tue Jan 8 17:36:27 2019 +0100

    readme: updated the build instructions

 README | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)
---
diff --git a/README b/README
index 5d9956ac..7757600a 100644
--- a/README
+++ b/README
@@ -119,15 +119,14 @@ Download
 
     http://ftp.gnome.org/pub/GNOME/sources/gthumb
 
-  * You can download and compile the latest development version using the 
+  * You can download and compile the latest development version using the
     following commands:
 
         git clone git://git.gnome.org/gthumb
         cd gthumb
-        mkdir build && cd build
-        meson ..
-        ninja
-        sudo ninja install
+        meson build
+        ninja -C build
+        sudo ninja -C build install
 
     More development information is available at
     https://wiki.gnome.org/Apps/Gthumb/development