docs/manual: fix graph-size documentation

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Cc: Thomas De Schampheleire <patrickdepinguin@gmail.com>
Acked-by: Thomas De Schampheleire <thomas.de_schampheleire@nokia.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

docs/manual/common-usage.txt

@@ -310,8 +310,8 @@ installed (+python-matplotlib+ on most distributions), and also the
 +argparse+ module if you're using a Python version older than 2.7
 (+python-argparse+ on most distributions).
 
-Just like for the duration graph, a +BR2_GRAPH_OUT+ environment is
-supported to adjust the output file format. See xref:graph-depends[]
+Just like for the duration graph, a +BR2_GRAPH_OUT+ environment variable
+is supported to adjust the output file format. See xref:graph-depends[]
 for details about this environment variable.
 
 .Note